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Replacing vented flat roof with unvented in wet winter

user-6813779 | Posted in Energy Efficiency and Durability on

I have a 50′ x 25′ flat roof on my house that needs replacing.

The builder in the late 70’s gave it a 4″ air gap between the top of the fiberglass batt insulation (R-19) and the roof sheathing. Rafters run in the 50′ direction and roof is white PVC in marine zone 4c. Behind the ceiling drywall below is poly sheet for an air barrier. Needless to say, the roof has never really dried out seasonally as it should. Layers like so (top to bottom):

– 1/2 Plywood
– 4″ Air Gap
– Fiberglass Bat
– Poly sheet
– 1/2″ Drywall

My plan is to tear off the roof sheathing, take out the fiberglass batts, and cut holes in the poly sheet (allowing the ceiling to dry to the interior of the house). Then, replace sheathing and blow in fiberglass to pack the ceiling completely (should be about R-30), seal sheathing seams, install 4″ reclaimed Polyiso on top (R-20ish), install a 1% taper polyiso on top of that (should lengthen the life of the roofing membrane since it will no longer have standing water), then a membrane on top (likely PVC, although TPO is also an option). Layers:

– 1% tapered Polyiso
– 4″ Polyiso
– 1/2″ OSB, seams taped for airtightness
– blown in fiberglass insulation (“dense packed”)
– 1/2″ Drywall

I think this ought to work well, but I have one major concern: are the wet rafters ever going to dry out to the interior of the house without causing massive mold/decay issues in the ceiling cavity?

Relative outdoor humidity right now is 80%+ and rain is pretty constant with a temp in the mid 40’s. The existing fiberglass batts are full of water which has been wicking into the rafters for 3-4 months. I wonder if I’m better off waiting for summer when humidity drops to 60% and there’s some sunlight and warmer temperatures to help dry things out?

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  1. GBA Editor
    Martin Holladay | | #1

    There are several questions here. The first step is to evaluate the condition of the rafters. They may be rotten. If you are unsure of your ability to evaluated the soundness of the rafters, hire an engineer to evaluate them. The worst-case scenario would be that the existing rafters are too soft to bear the roof weight, and will need to be entirely replaced.

    If an engineer tells you that the rafters are sound, it would be a good idea to remove the polyethylene from above by slitting the poly in each rafter bay, on the right and the left, with a utility knife, cutting the poly as close to the rafters as possible. In other words, cut it out and remove it -- don't just put a few holes in it.

    In your climate zone (Marine Zone 4), the type of roof you are contemplating needs to get at least 20% of the total R-value of the roof from the rigid foam layer, so it sounds like you will be safe in terms of the foam-to-fluffy ratio.

    For more information, see this article: Insulating Low-Slope Residential Roofs.

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